The growth and diversity of lightwave networks is placing new demands on optical technology. In particular, wavelength-division multiplexed (WDM) networks require new optical circuits to enable capacity management and provisioning, maintenance, and reliable and robust operation. These optical circuits include wavelength multiplexing and demultiplexing at terminal sites and wavelength monitors for determining system performance.
Recently, Add/Drop multiplexers have been used to extract selected wavelength channels in a WDM signal and to add replacement channels positioned at the same wavelengths as the dropped channels. The use of wavelength multiplexing/demultiplexing of WDM signals avoids the use of electronic demultiplexing circuits to extract and inject local electronic traffic in networks that use time-division-multiplexed (TDM) optical signals.
Add/Drop multiplexers and wavelength monitors use wavelength filters to select the desired wavelength to be processed. These wavelength filters can be implemented using distributed feedback reflectors or gratings. In a distributed feedback reflector or grating, it is known that periodic modulation in the index of refraction creates a stop band. Stop bands are ranges of frequencies where the reflection is near unity or the transmission is substantially zero. If this grating is divided or separated in the middle such that the resulting gap is a quarter wavelength in length or .pi./2 in phase, then the sharp transmission resonance appears in the middle of the stop band. This wavelength selective resonant grating structure, shown in FIG. 1, is commonly known as a quarter wave shifted grating [2,3]. (Note, the reference number(s) identified in brackets, e.g., [2,3], indicates the position of those references in the attached Appendix). The nature of the transmission resonance is the same if the gap is any odd multiple of .pi./2 in phase [4,5].
There is a continuing need for further improvements in wavelength selective resonant gratings for use as wavelength filters in the various components of optical networks.